Cosmic ray imaging and sensing are techniques that exploit the multiple Coulomb scattering of highly penetrating cosmic ray-produced muons to perform non-destructive inspection of materials without the use of artificial radiation. The Earth is continuously bombarded by energetic stable particles, mostly protons, coming from deep space. These particles interact with atoms in the upper atmosphere to produce showers of particles that include many short-lived pions, which decay into longer-lived muons. Muons interact with matter primarily through the Coulomb force, having no nuclear interaction and radiating much less readily than electrons. Such cosmic ray-produced particles slowly lose energy through electromagnetic interactions. Consequently, many of the cosmic ray produced muons arrive at the Earth's surface as highly penetrating charged radiation. The muon flux at sea level is about 1 muon per cm2 per minute.
As a muon moves through material, Coulomb scattering off of the charges of sub-atomic particles perturb its trajectory. The total deflection depends on several material properties, but the dominant effects are the atomic number, Z, of nuclei and the density of the material. The trajectories of muons are more strongly affected by materials that make good gamma ray shielding, such as lead and tungsten, and by special nuclear materials (SNM), such as uranium and plutonium, than by materials that make up more ordinary objects such as water, plastic, aluminum and steel. Each muon carries information about the objects that it has penetrated. The scattering of multiple muons can be measured and processed to probe the properties of these objects. A material with a high atomic number Z and a high density can be detected and identified when the material is located inside low-Z and medium-Z matter. In addition to muons, cosmic rays also generate electrons. Electrons are less massive and generally have lower momenta than muons and hence scatter more in a given material. Due to their larger scattering, electrons can be used to differentiate materials particularly those with low to medium Z and densities that may not significantly scatter muons.
Coulomb scattering from atomic nuclei in matter results in a very large number of small angle deflections of charged particles as they transit the matter. In some examples, a correlated distribution function can be used to approximately characterize the displacement and angle change of the trajectory that depends on the density and the atomic charge of the material. As an example, this distribution function can be approximated as a Gaussian distribution. The width of the distribution function is proportional to the inverse of the momentum of the particle and the square root of the real density of material measured in radiation lengths. The correlated distribution function of cosmic ray-produced particles (e.g., muons and electrons) can provide information on materials in the paths of the particles with no radiation dose above the Earth's background and proper detection of such cosmic ray-produced particles can be implemented in a way that is especially sensitive to selected materials to be detected such as good radiation shielding materials.
In some examples of cosmic ray imaging and sensing, a muon tomography system can be configured to perform tomography of a target object under inspection based on scattering of cosmic ray particles by the target object. For example, cosmic ray tomography systems can be used for detecting certain targeted objects, e.g., such as materials that can be used to threaten the public, including smuggled nuclear materials. Cosmic ray tomography detector systems can be used jointly with or an alternative to other nuclear material detectors such as gamma or X-ray detectors. Gamma and X-ray detectors operate by directing Gamma and X-ray radiation to a target and measuring penetrated Gamma and X-ray radiation. Shielding of nuclear materials can reduce the count rates in the Gamma and X-ray detectors and reduce the detection performance of Gamma and X-ray detectors. Cosmic ray tomography detection systems can be configured to detect shielded nuclear materials and objects.
In an example of a cosmic ray tomography detection system, the cosmic ray particle detectors can include arrays of drift-tube sensors configured to enable tomographic imaging of a volume of interest (VOI) using ambient cosmic rays as the illuminating radiation source. Cosmic ray charged particles, e.g., primarily muons and electrons, shower through the VOI, and measurement of individual particle tracks can be used to reconstruct the three-dimensional distribution of atomic number (Z) and density of materials in the VOI using particle scattering.